The present invention relates to the art of cryptography and, more specifically, to the implementation of improved techniques for achieving voice and/or data communication security over a radio and/or telephone channel.
Cryptography generally relates to the art of protecting sensitive communications against unauthorized access. The proliferation of electronic eavesdropping into sensitive police and military communications has spurred industry to create encryption/decryption devices which prevent such eavesdropping.
Techniques of encrypting or scrambling of radio or telephone signals can take on many forms. Scrambling can go from simple analog encryption to sophisticated digital encryption. Analog scrambling, which makes use of filtering schemes, inverting schemes and split-band audio schemes is generally easier to incorporate into the radio communication channel than digital scrambling, however, it is much easier for unauthorized users to unscramble an analog-scrambled voice than it is for them to unscramble digital-scrambled voice. Digital encryption, is more difficult to unscramble since it converts the voice to binary bits, scrambles the bits, then transmits the scrambled bits over the communication channel. Thus, the digital system makes unauthorized descrambling or deciphering difficult since all the intruding user "sees" is scrambled bit representation of voice and not scrambled voice itself.
Generally, the three functional blocks used in a digital speech encryption device are: (1) voice coder/decoder; (2) encryption/decryption algorithm, and (3) modulation/demodulation methodology. The voice coder/decoder block functions by first coding analog voice samples and then compressing those samples into a smaller number of binary bits. Thus, when combined with a suitable modulation methodology, the voice coder allows transmission over a communication channel of a smaller, specified bandwidth, and provides intelligible reproduction or decoding via the decode block on the receive end. The first generation voice coder/decoders were linear predictive coders (LPC-10) as discussed in Federal Information Processing Standards Publication (FIPS PUB) No. 137, U.S. Dept. of Commerce, NTIS, 5285 Post Royal Road, Springfield, Va. 22161, published on Nov. 28, 1984. Since FIPS PUB 137, other forms of voice coders arose such as: (1) continuously variable slope delta (CVSD) modulation, described in U.S. Pat. No. 4,167,700, (2) sub-band coding described in T. P. Barnwell et al., "A Real-Time Speech Sub-Band Coder Using the TMS 32010," IEEE Southcon. (1984), (3) hybrid sub-band coders described in U.S. Pat. No. 4,817,146, and (4) recent coders which include Code Excited Linear Prediction (CELP) Type Coders and Improved Multi-Band Excitation (IMBE) Voice Coders as described in Proceedings of ICASSP (1990), pp. 5-8, 177-180 and 465-468 . In addition, there are some very low bit rate laboratory voice coders being developed. An exemplary said voice coder is described in Y. J. Liu, "A High-Quality Speech Coder at 600 BPS", Proceedings of ICASSP (1990), pp. 645-648.
Although, voice coder/decoders of the conventional art can adequately interface with many different and suitable modulation/demodulation methodologies, typical voice coder/decoders must transmit large numbers of bits per second (BPS) in order to achieve commercial quality voice reproduction. For example, CVSD requires 12,000 BPS while sub-band coding and hybrid sub-band coding requires 9,600 BPS. However, in order to achieve suitable encryption/decryption over conventional channels, the selected coder/decoder must be combined with a modulation methodology that achieves adequate voice reproduction within the bandwidth restrictions of the particular channel.
The next functional block or, encryption/decryption block, is generally designed either to protect classified information or to protect sensitive but unclassified information. One conventional encryption technique used to prevent eavesdropping of sensitive but unclassified information is data encryption standard (DES). DES has become the standard algorithm used for sensitive, non-military applications such as, e.g., police communication. DES is fully explained in FIPS Pub. No. 46, U.S. Dept. of Commerce, NTIS, 5285 Post Royal Road, Springfield, Va. 22161, published on Jan. 22, 1988.
The third functional block or, modulation/demodulation methodology, is known in the art and can be described in M. Schwartz, Information, Transmission, Modulation and Noise, McGraw Hill, (3rd Edition, 1980); I. Korn, Digital Communications, (1st Edition, (2nd Edition, 1989). It is important when modulating or demodulating in a secure communication path, that efficient use be made of the frequency spectrum. Unless properly filtered conventional modulation techniques, such as frequency shift keying (FSK), minimum shift keying (MSK) or phase shift keying (PSK), occupy too wide a bandwith for the governmental standard of 15 KHz channels at the required data rates/hertz for all but the latest voice coders.
Conventional digital voice encryption devices which utilize DES standard encryption/decryption typically require a large frequency spectrum and often do not meet the government specified 15 KHz standard communications channel. Thus, new radio equipment must be purchased with government approval to allow deviation from the standard bandwidth in the digital mode, thereby forcing consumers to dispose of their older units. Also, conventional digital voice encryption systems may require linear response amplifiers and intermediate filters (IF), which again causes consumers to dispose of their older units.
A new digital radio standard is currently being devised by the government to achieve increased spectrum efficiency by narrowing the bandwidth in a portion of the spectrum to about 6.5 KHz, versus the current 15 KHz. Modulation methodologies such as 4-ary FSK, Generalized Tamed Frequency Modulation (GTFM), Quadrature Differential Phase Shift Keying (QDPSK), and pi/4 Shift QDPSK will be used to accomplish efficiency. These modulation techniques cannot achieve sufficient efficiency when designed into conventional radio transmitters without wholesale redesigning of the radio itself. This method will force consumers to purchase new radio equipment to obtain digital encryption capability.
Many consumers have older radios approved for the standard 15 KHz bandwidth channels which are not presently capable of being converted to include digital encryption capability. If the end user wishes to transmit data, as well as voice, conventional devices cannot automatically transmit and receive secure signals at numerous data and voice transmission rates. Nor can conventional devices permit the user to alternatively send a secure message through external Data Terminal Equipment (DTE) such as a keyboard, etc., or send a secure voice message at a slower speed for playback at the receiver at full speed. Furthermore, presently available devices have not been designed to automatically adapt the voice coder to a changing bit error rate (BER) environment, such as that found in a radio channel.